Cryogenic Air Separation Process

ABSTRACT

A cryogenic process of supplying oxygen to a power generation plant including at least an air separation unit ( 9,11 ), a liquid oxygen tank ( 15 ) and an air derived component liquid tank ( 17 ), comprises: During a first period: feeding a first air stream to the air separation unit at a first flowrate, feeding liquid oxygen from the liquid oxygen tank to the air separation unit, recovering a gaseous oxygen stream with a higher flow than the liquid oxygen stream from the air separation unit, sending at least one air derived component liquid to at least one air derived component liquid tank. During a second period: feeding the at least one air derived component liquid stream from the at least one air component liquid tank to the air separation unit, extracting a liquid oxygen stream from the air separation unit to the liquid oxygen tank, recovering a gaseous oxygen stream from the air separation unit and increasing the flowrate of the first air stream, feeding the air separation unit to a value greater than the first flowrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119(e) toprovisional application No. 60/795,143, filed Apr. 26, 2006, the entirecontents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a cryogenic air separation process.

All pressures listed in the document are absolute pressures.

Because of the global warming effect caused by the increased release ofCO₂ (carbon dioxide) generated by combustion processes, efforts havebeen made by utility companies and governments worldwide to reduce andminimize the CO₂ emission. One major source of CO₂ emission is the powergeneration plant's combustion process. There are mainly two types ofpower plants based on combustion processes: coal combustion and naturalgas combustion. Both of these processes produce CO₂ when generatingpower. The most efficient approach to reduce or minimize the CO₂emission is to capture most of the CO₂ emitted by the power plants. Forthis effort to be efficient, it must also target the existing coalcombustion plants that represent a large portion of the power generationplants worldwide. The oxy-combustion technique is quite advantageoussince it can be adapted to existing facilities as well.

A traditional power plant uses air as the source of oxidant (oxygen) tocombust the coal. Steam is generated by heating boiler feed water withthe hot combustion products (steam, CO₂, N₂, excess air). The steam isthen expanded in steam turbines to produce power. The combustionproduces CO₂ as a by-product, which, along with other gases such asresidual nitrogen of combustion air, moisture etc., constitutes the fluegas of the combustion. Due to the high content of nitrogen in air (78mol %), the CO₂ is very much diluted in the flue gas. To insure fullcombustion, the power plants must also run with an excess air ratio,which further dilutes the CO₂. The concentration of CO₂ in the flue gasof an air combustion plant is only about 10-20 mol %. The dilutedcomposition of CO₂ increases the size and the power consumption of theCO₂ recovery unit. Because of this dilution, it becomes very costly anddifficult to recover and capture the CO₂ especially with the lowpressure of the flue gas. Therefore, it is desirable to produce moreconcentrated CO₂ in flue gas, about 95 mol % of CO₂ purity is preferred,to minimize the abatement cost. An alternative technology for CO₂recovery from flue gas utilizes an amine contact tower to scrub out theCO₂. However, significant amount of heat is needed to regenerate theamine and to extract the CO₂ such that the amine process is not costeffective.

In order to avoid the dilution of CO₂ in N₂, the power generationindustry can switch to an oxy-combustion process: instead of utilizingair as oxidant, pure oxygen of 95% purity or better is used in thecombustion process. The combustion heat is dissipated in a recycled fluegas concentrated in CO₂. By doing so, since there is very small amountor almost no nitrogen in the system, it becomes possible to achieve aflue gas containing about 75-95 mol % CO₂, which is a significantimprovement over the previous 20 mol % of air combustion. The purity ofCO₂ in oxy-combustion's flue gas depends on the amount of air leakageinto the system and the purity of oxygen being utilized. An airseparation unit normally supplies the pure oxygen for combustion. Theflue gas rich in CO₂ exiting the boiler is cooled and treated to recoverthe CO₂ for subsequent disposal.

The potential main users of oxycombustion technology are existingpulverized coal power plants since an oxygen plant, a CO₂ recycle blowerand a CO₂ recovery from flue gas can be added to the existing plant toretrofit it so that the converted plant can comply with new CO₂ emissionstandard. New grass root plants are likely to base on cleaner IGCC.

For retrofitted oxycombustion coal plants, it is clear that the effortto capture CO₂ is hindered by the cost of the oxygen plant. Furthermore,the power consumption of the oxygen plant, which can be about 10% of thepower plant output, also introduces additional cost issues: part of thepower generated by the power plant must be diverted to supply the oxygenplant. Therefore, less power will be available to supply the grid,especially during peak demand when power is scarce and power costs arepremium, resulting in reduction of power plant's revenue. In thissituation, the economics of CO₂ capture, and disposal by oxycombustiontechnique, depend strongly on the cost and power consumption of theoxygen plant. Without an efficient setup for the oxygen generation, thecost penalty would be such that it would become uneconomical to operatethe clean and CO₂-free oxycombustion power plants.

In order to optimize the oxygen supply for oxycombustion scheme, severalstudies conducted by the power industry with the co-operation of variousoxygen suppliers have concluded that a low purity oxygen (about 95 mol%) is sufficient for oxycombustion and provides a low cost, low powerconsumption oxygen plant. This takes into account the impact of thepurity of CO₂ in flue gas caused by the oxygen purity on the subsequentCO₂ concentrating and purification equipment. However further costreduction of the cost of oxygen is needed to improve the economics ofthe CO₂ capture.

The Integrated Gasification Combined Cycle (IGCC) is a new highlyefficient power plant wherein, instead of performing the directcombustion of coal to generate hot flue gas for steam generation, thecoal is subjected to a partial oxidation process in which it is gasifiedto yield a mixture containing mostly of H₂ and CO called fuel gas. Thisfuel gas, after being treated to remove various pollutants or corrosivechemicals, is sent to a gas turbine where it is combusted to heat thecompressed feed air prior to the expansion. Pure oxygen of about 95%supplied by an air separation unit is used in the partial oxidationreaction of the coal gasifier. Since the CO of the fuel gas is combustedto yield CO₂ in the burner of the gas turbine, the resulting CO₂ is alsomixed with the nitrogen of the gas turbine feed air such that the CO₂recovery is also a costly and difficult task. To avoid difficult CO₂recovery due to a high flow, low pressure and much diluted CO₂ of thegas turbine, the fuel gas is subjected to a shift conversion wherein theCO reacts with steam to is produce H₂ and CO₂. The CO₂ can then berecovered economically by scrubbing with a solvent like in the Rectisolprocess. The fuel gas free of CO and CO₂ and containing mostly H₂ andsteam is then burned in the gas turbine to yield an almost CO₂-freeexhaust gas. In IGCC facilities, the oxygen plant is usually needed forthe partial oxidation portion, regardless of the need of the CO₂capture.

It can be seen from the above simple process description that pureoxygen gas, supplied by an air separation plant, is used in either thedirect combustion of an oxycombustion or the partial oxidation of anIGCC process. The production of oxygen requires additional capitalinvestment and consumes significant power to drive the compressionequipment. It is obvious that the power consumption of the oxygen plantand the oxygen plant cost must be optimized to reduce the impact of CO₂capture on the final cost of electricity. In addition to the oxygenplant, the CO₂ recovery from flue gas of an oxycombustion or from thefuel gas of an IGCC also consumes power and requires significantinvestment since the CO₂ must be further concentrated to about 95 mol %and then compressed to about 100 bar or higher for disposal.

This invention addresses the potential savings in power and cost of anoxygen plant integrated with an oxycombustion power plant for CO₂capture or an IGCC plant.

Power plants supply electricity to the grid. It is well known that powerdemand varies during the day, there are “peak” periods of high demand,hence high power cost, and there are “off-peak” periods of low demand,low power cost. Peaks usually occur in the daytime of the weekdays, forexample, from 9 AM to 5 PM. Off-peaks usually take place at nighttimefor example from 9 PM to 5 AM and week ends. There are also someintermediate demands and costs. The duration of peaks and off-peaks alsodepends on is the seasons, the variations of local temperature, theweather changes etc.

Because of the variable demand, power plants usually run at or near itsdesign capacity during peaks, but must idle at very low output duringoff-peaks. Off-peak demand can be as low as 15-20% of the ratedcapacity. Power cost is high during peaks and sometimes the utilitycompanies must purchase additional power from other suppliers to satisfydemand. The situation is reversed for off-peaks: power supply isabundant but demand drops sharply such that the power generatingequipment must be turned down to minimum and sometimes shut down.Utility companies encourage users to consume more power during off-peaksto avoid costly equipment shutdown by lowering the power cost sharplyfor off-peaks. The power cost for peak or high demand periods can be 3to 5 times higher than the power cost of the off-peak or low demandperiods.

The addition of an oxygen plant to the power generation plant worsensthe power cost structure especially for peak periods. Indeed, the oxygenplant must generate maximum oxygen flow to match with the maximum poweroutput; its power consumption is therefore at the highest during peaks.This additional power consumption is quite costly because it deprivesthe utility companies from having the available kW to sell on the gridat the premium value. As an indication, an oxygen plant consumes as muchas 10% of the power output of a power plant. During off-peaks, the poweroutput is at the minimum level; the consumption of the oxygen plant isalso at its lowest level and cannot take advantage of the lower powercost.

Therefore, there exists a need for an oxygen production process capableof tracking economically the demand curve of a power generation plantsuch that: the oxygen plant can minimize its power consumption duringpeaks while maintaining its supply of oxygen to the power plants at therated level. This reduction in power consumption will free up more kWfor the grid. The oxygen plant can maximize its power consumption duringoff-peaks to take advantage of the lower power cost while remainingcapable of supplying oxygen at reduced level. This high powerconsumption creates a power demand and keeps the power generatingequipment running above it minimum rate, thus potentially avoid costlyequipment shutdown.

SUMMARY OF THE INVENTION

According to this invention, there is provided a cryogenic process ofsupplying oxygen to a power generation plant comprising at least an airseparation unit, a liquid oxygen tank, and an air derived componentliquid tank, said process comprising:

a. During a first period:

-   -   i) feeding a first air stream to the air separation unit at a        first flowrate;    -   ii) feeding liquid oxygen from the liquid oxygen tank to the air        separation unit;    -   iii) recovering a gaseous oxygen stream with a higher flow than        the liquid oxygen stream from the air separation unit; and    -   iv) sending at least one air derived component liquid to at        least one air derived component liquid tank.

b. During a second period:

-   -   i) feeding the at least one air derived component liquid stream        from the at least one air component liquid tank to the air        separation unit;    -   ii) extracting a liquid oxygen stream from the air separation        unit to the liquid oxygen tank;    -   iii) recovering a gaseous oxygen stream from the air separation        unit; and    -   iv) increasing the flowrate of the first air stream, feeding the        air separation unit to a value greater than the first flowrate.

According to Further Optional Features:

The air separation unit produces substantially the same flowrate ofgaseous oxygen during the first and second periods.

The air separation unit produces a higher flowrate of gaseous oxygenduring the first period than in the second period.

The power costs are average during the second period and below averageduring a third period, wherein during the third period, the processincludes:

-   -   i) feeding the at least one air derived component liquid stream        from the at least one air component liquid tank to the air        separation unit;    -   ii) extracting a liquid oxygen stream from the air separation        unit to the liquid oxygen tank;    -   iii) recovering a gaseous oxygen stream from the air separation        unit;    -   iv) increasing the flowrate of the first air stream feeding the        air separation unit to a value greater than its flowrate in the        first period; and    -   v) wherein the flowrates of the first air stream in the second        period and the third period are substantially equal.

The power costs of the first period are higher than average.

The power costs of the second period are average or lower than average.

The power costs of the first period are higher than average and thepower costs of the second period are average or lower than average.

The power demand of the first period is higher than average.

The power demand of the second period is average or lower than average.

The power demand of the first period is higher than average and thepower demand of the second period is average or lower than average.

The power generation plant is an oxycombustion plant.

The power generation plant is an IGCC plant.

At least one air derived component liquid is liquid nitrogen and whereinstep iv) of period a) of Claim 1 comprises removing liquid nitrogen froma column of the air separation unit.

At least one air derived component liquid contains 80 mol % nitrogen orgreater.

At least one air derived component liquid is liquid air.

At least one air derived component liquid contains 35 mol % oxygen orgreater wherein step iv) of period a) of Claim 1 comprises removingliquid nitrogen from a column of the air separation unit.

BRIEF DESCRIPTION OF THE INVENTION

Illustrative embodiments of the invention are described below. While theinvention is susceptible to various modifications and alternative forms,specific embodiments thereof have been shown by way of example in thedrawings and are herein described in detail. It should be understood,however, that the description herein of specific embodiments is notintended to limit the invention to the particular forms disclosed, buton the contrary, the intention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims.

It will of course be appreciated that in the development of any suchactual embodiment, numerous implementation-specific decisions must bemade to achieve the developer's specific goals, such as compliance withsystem-related and business-related constraints, which will vary fromone implementation to another. Moreover, it will be appreciated thatsuch a development effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking for those of ordinary skill in theart having the benefit of this disclosure.

The invention will be described in greater detail with reference to thefigures, wherein FIG. 1 represents the Prior art approach in which theair separation unit simply supplies the oxygen to the power plant. FIGS.2 and 3 represent an air separation unit operating according to theinvention at different periods, FIGS. 4 and 5 represent different phasesof operation of plant according to the prior art and FIGS. 6, 7, 8 and 9represent air separation units capable of operating according to theinvention.

As shown on FIG. 1 for the Prior art, during peaks in power demand, 1000Nm3/h of feed air 6 is treated to yield 200 Nm3/h of oxygen 12 requiredfor peak demand by power generation plant 10. If the demand is reduced,less air is sent to the oxygen plant 13 to yield less oxygen. The airflow is essentially proportional to the oxygen demand.

As shown on FIG. 2 for the bascule approach, during peaks in powerdemand, 110 Nm3/h of liquid oxygen 53 from a liquid oxygen tank 15 isfed to the oxygen plant 13, vaporized and combined with the oxygenproduced from the feed air to yield 200 Nm³/h of a high oxygen stream 12required for peak demand by the power generation plant 10. The recoveredrefrigeration from the vaporization of liquid oxygen is used to liquefyliquid nitrogen 49 and to store it in a liquid nitrogen tank 17. Since aportion of the oxygen is provided by vaporizing the liquid oxygen, theair flow 6 to the oxygen plant can be reduced by about the sameproportion to 450 Nm³/h resulting in significant power reduction whilemaintaining the total rated flow of oxygen to satisfy the peak demand.

As shown in FIG. 3, during average periods, 900 Nm³/h of feed air 6 aresent from compressor 1 to the oxygen plant 13 to produce 150 Nm³/h ofoxygen 45. 30 Nm³/h of liquid oxygen are sent from the oxygen plant tothe oxygen tank 15 whilst liquid nitrogen 49 is sent from the nitrogentank 17 to the oxygen plant.

As shown in FIG. 3, during off-peaks, instead of being reduced, the airflow 6 is at 900 Nm³/h to produce more oxygen than the demand. 80 Nm3/hof the excess gaseous oxygen is liquefied by feeding and vaporizing theliquid nitrogen 49 produced during peaks. The produced liquid oxygen 53is then stored in the liquid oxygen tank 15 to restore the oxygeninventory which will be needed in the subsequent peak periods.

In order to illustrate this concept, a simple model of power demand (orgeneration rate) can be used:

Power generation: Oxygen Power Cents (US$) requirement/ Demand per kWhNm³/h Peak (⅓ of the time) 100% 4.5 200 Average (⅓ of the 75% 3 150time) Off-peak (⅓ of the 50% 1.5 100 time)

The performance of an Air Separation Unit (ASU) can be approximated by asimple oxygen recovery ratio of about 20%: for 1000 Nm³/h feed air tothe ASU, the corresponding recovery rate of oxygen is 200 Nm³/h. For lowpurity, low pressure oxygen for oxycombustion application, the powerconsumption of the oxygen plant is mainly the power consumption of theair feed compressor hence the air feed flow.

The prior art process consists of a basic Air Separation Unit, itsoxygen production output is adjusted simply by adjusting the air feedflow to the unit. As an approximation, the power consumption is assumedto be proportional to the feed air flow.

Let us compare the air feed rate of this new process with the prior artprocess:

New Process Prior Art Case 1 Air Power Power Daily basis Cents/kWh FlowCost Air Flow Cost Peak 100% Power - 4.5 450 16200 1000 36000 8 hoursAverage 75% 3 900 21600 750 18000 Power - 8 hours Off-peak 50% 1.5 90010800 500  6000 Power - 8 hours Total 48600 60000 (−19%) Note: Assumingthe air compressor can be turned down to 50% (for example by using 2compressors in parallel)It can be seen from above table that:

a) The maximum air flow of the new ASU is 90% of the maximum air flow ofthe prior art. This represents smaller equipment and a reduction ofplant cost.

b) The cost of power to operate the new process is reduced by about 19%based on the above model. This is a significant cost reduction. Theeconomics of oxycombustion and CO₂ capture in particular can thereforebe improved.

c) During peak periods, the power consumption of the oxygen plant issharply reduced by 55%; this represents an important availability ofpower to supply the demand of the grid. If demand cannot be satisfied,utility companies usually have to purchase additional power from anothernetwork at a very high cost. This reduction of consumption of the oxygenduring peaks can alleviate the situation and will result in majorsavings for utility companies.

The concept can also be applicable to situations where the demandremains constant throughout the high demand periods (highest power cost)or low demand periods (lowest power cost). In this situation, duringhigh demand, the air flow to the ASU is reduced to the limit ofmachinery's turndown to minimize its power consumption. During lowdemand, the air flow is increased not only to satisfy the demand butalso to produce liquid oxygen to be vaporized during the periods whenpower cost is high.

New Process Prior Art Case 2 Air Power Power Daily basis Cents/kWh FlowCost Air Flow Cost Peak 100% Power - 4.5 600 21600 1000 36000 8 hoursAverage 100% 3 1200 28800 1000 24000 Power - 8 hours Off-peak 100% 1.51200 14400 1000 12000 Power - 8 hours Total 64800 72000 (−10%)

Since the oxygen plant must supply a constant oxygen flow and theadditional oxygen liquid for the high demand periods, the plant size forthe off-peaks in this example must be increased by 20%. However, thesaving achieved is still significant at 10% and the ASU power can be cutback by 40% during the periods of high demand.

This new process can be used to minimize the plant cost and stillprovides significant power cost saving. Indeed, in the above example ofCase 1 we can arrange to have the following configuration:

New Process Prior Art Case 3 Air Power Power Daily basis Cents/kWh FlowCost Air Flow Cost Peak 100% Power - 4.5 750 27000 1000 36000 8 hoursAverage 75% 3 750 18000 750 18000 Power - 8 hours Off-peak 50% 1.5 750 9000 500  6000 Power - 8 hours Total 54000 60000 (−10%)

As can be seen, the air flow can be kept constant and the oxygen demandcan vary during the peaks and off-peaks. This strategy results in 25%reduction in plant size while preserving a good 10% reduction in powercost.

A similar approach can be used to estimate the savings in some othermodels and the concept appears to be advantageous in most situations.

It is useful to note that by liquefying liquid nitrogen when liquidoxygen is vaporized, or vice versa by liquefying liquid oxygen whenliquid nitrogen is vaporized, we can recover and store the refrigerationunder the liquid form such that there is no major power expenditure toliquefy these important amounts of liquid involved in the transfers.

The above example use liquid nitrogen as a means to transfer and storethe refrigeration during periods of peaks and off-peaks. The process canbe applicable to a liquid of another composition derived from air suchas liquid air, a liquid rich in oxygen (greater than 35% O₂) or a liquidrich in nitrogen (greater than 80% N₂). Two or more liquid streams canalso be used if needed, for example, during peaks, liquid oxygen is fedand vaporized in the ASU, a stream of liquid N₂ and a stream of liquidair can be extracted from the ASU to compensate for the refrigeration.

The term “bascule” is used to describe the cryogenic air separationprocess in which, in one phase, a first liquid stream is used to liquefyan oxygen stream. In a next phase, liquid oxygen produced is then fed tothe process to allow extraction and restoration of the first liquidstream. Since the process simply exchanges refrigeration between liquidoxygen and the first liquid stream, it does not require power intensiveequipment to liquefy a gaseous stream like in traditional liquefactionequipment.

In the new invention, during low demand periods, the ASU can increasethe air feed to restore the liquid oxygen inventory by re-feeding theliquid nitrogen produced in the high demand periods back into thesystem. The higher feed air coupled with low power cost can provide anadded advantage: some small amount of liquid can be extracted from thecryogenic cold box of the oxygen plant with almost no power or costpenalty, for example by simply increasing the flow of a cold box'sexpander. This additional liquid can be fed back to the cold box duringpeaks, reducing the need to operate the cold box's expander(s) duringpeaks thus increasing the efficiency and ability of the system to bettertrack the demand. The cryogenic oxygen plant may be equipped with coldcompression equipment, which consumes refrigeration. Such small amountof additional liquid generated inexpensively during off-peaks, coupledwith the liquid resulting from refrigeration exchange of the bascule,can improve the cold requirement of the system during peaks. Therefore,the cold box's expander(s) can be throttled or even shut down to furthercut back the air flow and still be able to maintain good efficiency ofthe distillation columns and satisfy the refrigeration need of coldcompression equipment, thus increasing the saving of the bascule.

Oxygen plants equipped with “bascule” features have been utilized in theindustry for some time. However, this usage has been limited to thetracking of the usage demand of the clients of the oxygen plant,independently of the power demand of the utility companies like theobject of this new invention. In another word, the bascule has beenapplied previously to the client side of electricity business, howeverthis new invention addresses the integration of the bascule oxygen plantto the supply side, and in particular to the power generation aspectcreated by the need of oxycombustion or the partial oxidationrequirement of IGCC plants.

In prior art, the economics of an air separation plant can also beimproved by liquefying a first liquid stream during the off-peak periodswhen power cost is low. When power cost is high, the liquid is thenvaporized in the air separation unit (ASU) allowing reducing the airflow to minimize the power. The basic difference between this techniqueand the present invention is that the excess of refrigeration produce byvaporizing the liquid is mainly used to compress a cold gas stream ofthe ASU at cryogenic temperature to higher pressure, and not to recoveran equivalent liquid flow for subsequent use. Power intensiveliquefaction equipment such as high pressure compressors and additionalgas expanders must be provided to run the liquefaction unit during offhours. The liquefaction equipment can be integrated with the oxygenplant. This prior art is illustrated schematically in FIGS. 4 and 5.

FIG. 4 shows the high power demand phase wherein nitrogen compressed ina cold compressor 16 is sent to the power plant's gas turbine, a portionof this cold compressed nitrogen can be optionally recycled to the airseparation unit 13 to improve the distillation or to vaporize liquidoxygen, and liquid air from storage tank 18 is sent to the airseparation unit.

FIG. 5 shows the low power demand phase wherein air is sent via a warmcompressor to the power generation plant 10. Liquid air formed inliquefier 14 is sent to liquid air storage tank 18.

It is clear the concept of this invention can use a combination of bothtechniques: a bascule feature and some ability to generate additionalliquid during low demand and low power cost. The added liquid can, forexample, be fed back to the system during peaks to enable an economicalcold compression of gaseous nitrogen to higher pressure for the IGCC'sgas turbine injection to lower the compression power requirement in thepeak demand period.

In summary, all power plants are subjected to daily usage variations andthis variable characteristic can be utilized advantageously by thebascule approach of the new invention such that the cost of oxygensupply for oxycombustion power plants can be minimized. The concept isdirectly applicable to IGCC plants.

FIG. 6 shows an air separation unit capable of operating according tothe invention. The plant uses a double column with a medium pressurecolumn 9 operating at around 3.6 bar to 4.0 bar and a low pressurecolumn in a dual reboiler configuration operating at around 1.32 bar.

For a normal run, air is compressed in compressor 1 and purified inpurification unit 5. The air is then cooled in exchanger 7 as stream 6and sent in essentially gaseous form to column 9.

Oxygen enriched liquid 19 is sent from the medium pressure column 9 tocolumn 11. Medium pressure nitrogen is used to reboil condenser 21 at anintermediate location between oxygen enriched feed and the bottom ofcolumn 11. Part of the medium pressure nitrogen 25 is compressed bymotor-driven compressor 27 and used to reboil condenser 29 at the bottomof column 11. The liquid formed is expanded in valve 22 and sent back tothe top of the column 9. The condensed medium pressure nitrogen is usedas reflux 35 for column 9, reflux 51 for column 11 and feed 49 fornitrogen tank 17. A stream of nitrogen 37 at the pressure of column 9 issent to the exchanger 7 where it warms and is then sent to turbineexpander 39 where it is expanded and then fully warmed in the exchanger7 to form waste stream 43. Product oxygen 45 is withdrawn as a gas froma section between the two reboilers 21, 29. Low pressure nitrogen iswarmed in exchanger 7 and exits as stream 47.

During high power demand, the expander 39 does not function or sees itsflow sharply reduced. Liquid nitrogen is sent to the tank 17 as stream49 and liquid oxygen 53 is sent from tank 15 to the bottom of column 11wherein it vaporizes.

The air flow is reduced by reducing the flow of compressor 1.

Because of the wide flow fluctuations of the nitrogen expander invarious modes, it is not practical to use the power generated by thenitrogen expander to drive the cold compressor 27. Indeed, in peak mode,the duty required by the cold compressor is very high to vaporizemaximum flow of oxygen, meanwhile the flow of the expander is sharplyreduced or even zero such that there is not sufficient power of theexpander to drive any equipment. Therefore an electric motor is a properchoice to drive the cold compressor.

During low power demand, the expander 39 functions at or near its peak.Liquid nitrogen is sent from the tank 17 to section 11 as stream 49 andliquid oxygen 53 is sent to tank 15 from section 31.

The process according to the invention could of course be operated usingother types of apparatus, for example that of FIG. 7 wherein the oxygenfrom section 31 is optionally pumped by pump 32 and then vaporized in anexternal exchanger 50. Part of the cold compressed medium pressurenitrogen 55 condenses in exchanger 50 to provide the necessary heat forvaporization of oxygen. In this arrangement, the cold compressor 27provides the pressurized nitrogen needed for condensation in bothexchangers 50 and 29. One can also opt to further compress stream 55 byanother cold compressor (not shown) should the required pressure ofoxygen stream 45 be higher.

Another embodiment is shown in FIG. 8: air to vaporize the oxygen isproduced by booster 8, which compresses about a quarter of the feed airflow. This air condenses in exchanger 7 against the vaporizing liquidoxygen of stream 45 extracted from the bottom of the column.

Liquid oxygen from storage 15 can be fed to the column 11 or directly anexternal vaporizer without passing to the column. It can also bevaporized in the exchanger 50, 7 or another exchanger and the resultinggaseous oxygen is mixed with gaseous oxygen produced by the column.

FIG. 9 shows a variant of FIG. 7 in which all the air is compressed to asingle pressure in compressor 1, purified in purification unit 5 andsent to the column 9 as gaseous stream 6. The rest of the figure is asin FIG. 7 except that there is no longer a liquid air reflux stream sentfrom column 9 to column 11.

For all figures, the apparatus uses a single turbine, that turbine beinga high pressure nitrogen turbine.

1. A cryogenic process of supplying oxygen to a power generation plantcomprising at least an air separation unit, a liquid oxygen tank and anair derived component liquid tank, said process comprising: a. During afirst period: i) feeding a first air stream to the air separation unitat a first flowrate; ii) feeding liquid oxygen from the liquid oxygentank to the air separation unit; iii) recovering a gaseous oxygen streamwith a higher flow than the liquid oxygen stream from the air separationunit; and iv) sending at least one air derived component liquid to atleast one air derived component liquid tank. b) During a second period:i) feeding the at least one air derived component liquid stream from theat least one air component liquid tank to the air separation unit; ii)extracting a liquid oxygen stream from the air separation unit to theliquid oxygen tank; iii) recovering a gaseous oxygen stream from the airseparation unit; and iv) increasing the flowrate of the first airstream, feeding the air separation unit to a value greater than thefirst flowrate.
 2. Process according to claim 1 wherein the airseparation unit produces substantially the same flowrate of gaseousoxygen during the first and second periods.
 3. Process according toclaim 1 wherein the air separation unit produces a higher flowrate ofgaseous oxygen during the first period than in the second period. 4.Process according to claim 1 wherein the power costs are average duringthe second period and below average during a third period, whereinduring the third period, the process includes: i) feeding the at leastone air derived component liquid stream from the at least one aircomponent liquid tank to the air separation unit; ii) extracting aliquid oxygen stream from the air separation unit to the liquid oxygentank; iii) recovering a gaseous oxygen stream from the air separationunit; iv) increasing the flowrate of the first air stream feeding theair separation unit to a value greater than its flowrate in the firstperiod; and v) wherein the flowrates of the first air stream in thesecond period and the third period are substantially equal.
 5. Processaccording to claim 1 wherein the power costs of the first period arehigher than average.
 6. Process according to claim 1 wherein the powercosts of the second period are average or lower than average.
 7. Processaccording to claim 1 wherein the power costs of the first period arehigher than average and the power costs of the second period are averageor lower than average.
 8. Process according to claim 1 wherein the powerdemand of the first period is higher than average.
 9. Process accordingto claim 1 wherein the power demand of the second period is average orlower than average.
 10. Process according to claim 1 wherein the powerdemand of the first period is higher than average and the power demandof the second period is average or lower than average.
 11. A processaccording to claim 1, in which the power generation plant is anoxycombustion plant.
 12. A process according to claim 1, in which thepower generation plant is an IGCC plant.
 13. A process according toclaim 17 in which at least one air derived component liquid is liquidnitrogen and wherein step iv) of period a) of claim 1 comprises removingliquid nitrogen from a column of the air separation unit.
 14. A processaccording to claim 1, in which at least one air derived component liquidcontains 80 mol % nitrogen or greater.
 15. A process according to claim1, in which at least one air derived component liquid is liquid air. 16.A process according to claim 1, in which at least one air derivedcomponent liquid contains 35 mol % oxygen or greater wherein step iv) ofperiod a) of claim 1 comprises removing liquid nitrogen from a column ofthe air separation unit.
 17. A process according to claim 1 wherein inat least one of step a) ii) of claim 1 the liquid oxygen is fed to acolumn of the air separation unit.
 18. A process according to claim 1wherein in step a) ii) of claim 1 the liquid oxygen is fed to anexchanger of the air separation unit without passing via a column of theair separation unit.
 19. A process according to claim 4 wherein in atleast one of step i) of claim 4 the liquid oxygen is fed to a column ofthe air separation unit.
 20. A process according to claim 4 wherein instep i) of claim 4 the liquid oxygen is fed to an exchanger of the airseparation unit without passing via a column of the air separation unit.